Slide 42 - CSS Recommendations from the W3C § Session 03 - HTML, Introduction to CSS

CSS Recommendations from the W3C

CSS 1, December 1996; Revised January 1999)
CSS 2, May 1998
CSS 2.1, June 2011
CSS 2.2, September 2015
CSS Current Work (CSS level 3)

CSS Validator

W3C CSS Validation Service
http://jigsaw.w3.org/css-validator/